NAMS 2002 Workshop - ICOM 2008

Horizontal shrinkage does not have an effect as high as that of vertical
shrinkage. This is due to the different interaction of the layers in each direction,
which leads to different levels of accumulation of these effects. As a rule of
thumb, the relative vertical shrinkage is uniform, whereas the lateral one is not.
This happens because when shrinking, each layer is impeded by the layers
directly above it which have already solidified.
Coagulation baths with increasing NMP to water ratios were tried, finding
minimum values for macrovoid suppression. Their effect on the solidification of
the membrane was also assessed. The inclusion of a vapor bath before
immersion precipitation was studied for creating a skin that slowed down the
exchange of solvent and non-solvent. For systems where a critical casting
thickness can be found, it has been seen that macrovoids tend to accumulate
inside the features, where the membrane is thicker.
Tunability of the porosity of the membrane is an interesting feature since the
inner structure of the membrane is also of high importance for the different
applications. Therefore, a methodical study of the PES/PVP/NMP/water system
was carried out. Variations in the compositions of both the casting solution and
the coagulation bath were tested for influence on the thickness, extent of
shrinkage and porosity of the final membrane. This has shown that the final
morphology of the membrane can be modified, including the appearance of the
skin layer.
The study of PSuF has clarified several aspects of the phase separation process.
The tests we have performed have provided us with tools to foresee and work
around potential challenges when patterning membranes for future applications.